Delayed effects of acute oral and chronic inhalational exposure to methylparathion on learning and memory in rats.

Impairment of acquisition phase of the learning process was observed in rats even at 3 weeks after single oral exposure to the near-lethal dose of commercial grade methylparathion (MP) followed by atropine resuscitation. Though there was a trend towards memory impairment in this group of animals, memory retention was not significantly affected. Chronic inhalational exposure to MP (one exposure/day for 3 weeks) did not significantly alter learning or memory. No significant alteration in red blood cell or brain acetylcholinesterase levels were observed in either of the two groups. It appears that behavioural effects can persist even 3 weeks after exposure to acute near-lethal doses of the pesticide, as occurs in the clinical situation of suicidal attempts; while repeated exposure to no-observed-effect-level doses as occurs in farm and factory workers, may not be associated with behavioural changes.